The National Gallery is unveiling its refurbished main entrance in Trafalgar Square after building work which took more than a year at a cost of £20m.
The ceiling has been stripped back to its former glory
The redevelopment has transformed a restricted and cramped entrance hall into "a grand and dramatic open area", the London gallery said.
It is being presented to the press on Wednesday and will reopen to the public on Saturday.
The gallery was visited by almost five million people last year.
The 19th Century decorative ceiling by JD Crace in the staircase hall has been restored to its former glory, with white over-painting stripped off to reveal the original decoration.
Casings to the staircase have been removed to reveal pink wall marble, sourced from quarries in Tunisia.
Designed by William Wilkins and originally opened in 1838, it is the most visited museum or gallery in the UK.
The first major expansion was in the 1870s, with further work in the 1880s, 1911 and the 1920s. The north galleries were added in 1975 and the Sainsbury Wing opened in 1991.
Gallery director Charles Saumarez Smith said: "The scheme provides the physical and experiential introduction that the Wilkins building, the paintings and our visitors deserve."